Embodiments of the present invention relate to methods for measuring interfacial or surface tension of a first fluid in contact with a second fluid.
Many methods are known that can measure the interfacial tension between two immiscible liquids. Likewise many methods are known for the measurement of the surface tension of a liquid in a gas.
However some liquids contain a variety of dissolved species, which can migrate towards a newly-formed surface or interface with another fluid. For such liquids the surface tension or interfacial tension may change with time, often over short timescales, as the dissolved species migrate towards the newly-formed surface. For example the liquid may contain surface active species, which can migrate to a newly-formed surface and even diffuse across into the other phase. Such species can alter the value of the interfacial or surface tension over time as the species migrates to the surface, this is particularly the case if the dissolved species are surface-active.
For such liquids it is therefore possible to distinguish between a newly-formed interfacial or surface tension, which exists at the moment of formation of a new interface or surface, and an aged interfacial or surface tension, which is arrived at once an equilibrium state is achieved. Additionally it may be of interest to monitor the change in interfacial or surface tension as the surface ages, to obtain information about the dynamics.
However, classic methods of measuring interfacial tension or surface tension tend to rely on obtaining an equilibrium state before measuring the interfacial tension. These methods are therefore not suitable for measuring the evolution of the interfacial or surface tension during short timescales. Such known methods include the Wilhelmy plate method, the de Nouy ring method and the Pendant Drop method. All these methods rely on comparing a gravitational force effect, produced by a difference in density, and inferring the interfacial tension or surface tension. The pendant drop method relies on knowing the densities of the fluids involved, in order to know the gravitational forces at work. The Wilhelmy plate and de Nouy ring are typically used for surface tension measurements, and involve nulling the mass of the probe, such that only an addition force due to surface tension is measured.
More elaborate methods of measuring interfacial and surface tension exist that do not rely on gravitational inference. One known method of measuring interfacial tension or surface tension over short timescales is the Maximum Bubble Pressure method. In this method a capillary containing a first fluid is inserted into a second fluid and the first fluid is pressurized to form a droplet or bubble at the exit of the capillary tube. The maximum pressure required to generate the bubble or droplet indicates the interfacial or surface tension. Furthermore, different rates of generation of the droplets or bubbles allows the measurement of interfacial tension or surface tension for differently aged surfaces to be measured, thus giving an indication of the evolution of the interfacial or surface tension.
However, one problem with this method is that it involves the formation of a three-phase contact line between the two fluids and the end of the capillary tube. Thus, any surface contamination on the end of the capillary tube can influence the result. When being used to measure model systems, where the measurement can be kept clean, this may not be a problem. However, when measuring real systems, for example fluids extracted from the ground—which may comprise a mixture of water, oil and a wide variety of dissolved species—contamination may become a major factor in influencing the result, rendering accurate measurement impossible.
Microfluidic devices have also been employed to measure interfacial tension. However, these devices also involve contact with a solid surface, introducing contamination issues as discussed above.
PCT patent application, WO 2009/125119, describes measuring the interfacial tension between two fluids in a microfluidic environment. The method relies on generating a map of transition points that mark the breakdown of a stream of one fluid into discrete droplets. This method does not involve contact with a surface and so overcomes the problem of surface contamination. However, as the method of measuring interfacial tension relies on the breakdown of a defined stream into a series of droplets, it is not possible to also measure the aged interfacial tension. Rather, the interfacial tension defines the breakup time, and therefore the time of the measurement.
Therefore, there remains a need in the art for a method of measuring interfacial and surface tension that is insensitive to contamination and can provide dynamic measurements for a newly formed interface through to an aged interface.